It is desirable that the output speed of an electric motor may be varied. For example, in large commercial refrigeration applications where three phase motors are used to drive compressors, there is a relatively constant output of the compressor if it is coupled directly to the motor, and this is not desirable if the demand decreases. In addition a compressor, which is directly coupled to the output shaft of a three phase motor, has a high starting torque (which adversely affects the life of the associated machinery) and also has a high initial power consumption during startup.
While variable speed electric motors are available, as a practical matter they are not available for the higher horsepower outputs required for commercial refrigeration installations.
In order to provide a variable output for a constant speed electric motor of the three phase type, the output shaft of the electric motor could be coupled to the variable speed transmission, such, as for example, a traction drive transmission, an impulse drive transmission or a controlled differential drive transmission. Examples of the latter form of transmission are shown in U.S. Pat. Nos. 2,762,239 and 2,897,691. The combination of a separate and variable speed transmission coupled to the output shaft of a constant speed motor has several disadvantages, such as large size and complexity. Also, such a unit would not be suitable for a hermetically sealed compressor unit.